1. Field of the Invention
The present invention relates to digital data transmission method and apparatus for transmitting a band-compressed digital signal such as a picture signal or an audio signal to record and reproduce the digital signal.
2. Description of the Prior Art
2.1. Previously Proposed Art
A digital data transmission apparatus has been utilized to transmit a band-compressed digital information signal such as a picture signal or an audio signal. FIG. 1 is a block diagram of a conventional digital data transmission apparatus. As shown in FIG. 1, a conventional digital data transmission apparatus 11 comprises a transmitting system 12 for processing a digital picture signal representing a digital information signal, a transmission path 13 for transmitting the digital picture signal processed in the system 12, and a receiving system 14 for processing the digital picture signal transmitted through the transmission path 13. The processing in the transmitting system 12 and the processing in the receiving system 14 are complementary to each other.
A digital picture signal designating a frame of picture data Dp is input to a signal dividing section 15 of the system 12. Thereafter, as shown in FIG. 2(a), the digital picture signal is divided into pieces of partial picture data DPi (i=1,2, - - - , the same shall apply hereinafter). The partial picture data DPi are respectively composed of 64 pixels (8 pixels in a lateral direction .times.8 pixels in a longitudinal direction) to be orthogonally converted and are allocated in a plurality of encoded blocks BEi. Thereafter, the partial picture data DPi are transferred to an orthogonal converting section 16 and are orthogonally converted into pieces of converted picture data in each of the encoded blocks. For example, a discrete cosine transformation is performed for each of the partial picture data DPi. In this case, energy is concentrated on lower frequency components of the converted picture data according to statistical properties of the picture signal. Thereafter, the converted picture data are transferred to a quantizing and variable-length encoding section 17.
In the section 17, the converted picture data are adaptively quantized to produce pieces of quantized data according to a predicted dynamic range and a predicted length of data obtained by quantizing and encoding the converted picture data in variable-length. Thereafter, the quantized data are encoded in variable-length and are encoded in run-length in the section 17. Therefore, a total length of the converted picture data is reduced in the section 17. In other words, the converted picture data are compressed to produce pieces of compressed picture data DCi allocated in a plurality of compressed blocks BCi in the section 17. Thereafter, the compressed picture data DCi are transferred to a synchronizing-identifying-error correcting codes adding section 18.
In the section 18, as shown in FIG. 2(c), a group of compressed picture data DCi is equally divided into pieces of divided picture data which each has the same synchronizing data length. The divided picture data are allocated in a plurality of synchronizing blocks BSi which are formed according to the same format. Also, a synchronizing signal SSi, an identifying code IDi, an address pointer PAi and a block address such as a block number NBi are added in a synchronizing signal region RSi, an identifying code region RIi, an address pointer region RPi and a block number region RBi which are placed in a head portion of each of the synchronizing blocks BSi. Also, an error correcting code ECi is added in an error correcting code region REi placed in a rear portion of each of the synchronizing blocks BSi. Therefore, pieces of synchronized picture data DSi respectively composed of the synchronizing signal SSi, the identifying code IDi, the address pointer PAi, the block number NBi, a piece of divided picture data and the error correcting code ECi are allocated in the synchronizing blocks BSi. Thereafter, the synchronized picture data DSi are recorded in a recording medium 19 through the transmission path 13. Therefore, the compressed picture data DCi with the synchronizing signal SSi, the identifying code IDi, the address pointer PAi, the block number NBi and the error correcting code ECi are recorded in the recording medium 19.
In cases where the compressed picture data DCi are reproduced from the recording medium 19, the synchronized picture data DSi recorded are transmitted to a synchronizing-identifying-codes separating and error correcting section 20 of the receiving system 14 through the transmission path 13. In this case, a transmission error occurs in the compressed picture data DCi of the synchronized picture data DSi at a prescribed probability. Therefore, the transmission error is deleted in the section 20. In detail, the synchronizing blocks BSi are recognized according to the synchronizing signals SSi and the identifying codes IDi, and the compressed picture data DCi included in the synchronizing picture data DSk are recognized according to the address pointer PAk and the block number NBk. Thereafter, an error correction is performed to delete the transmission error occurring in the compressed picture data DCi of the synchronizing picture data DSk according to the error correcting code ECk. Thereafter, the synchronizing signals SSi, the identifying codes IDi, the address pointers PAi, the block numbers NBi and the error correcting codes ECi are separated from the synchronized picture data DSi to correctly reproduce the compressed picture data DCi. Thereafter, the compressed picture data DCi are transferred to a variable-length decoding and inverse quantizing section 21 in sequence. In the section 21, the compressed picture data DCi coded in the variable-length are decoded and inversely quantized to reproduce the converted picture data in sequence. Thereafter, the converted picture data reproduced are transferred to an inverse orthogonal converting section 22 and are inversely converted according to an inverse orthogonal conversion to reproduce the partial picture data DPi. Thereafter, the partial picture data DPi reproduced are transferred to a signal synthesizing section 23, and the digital picture signal designating the picture data Dp are reproduced by synthesizing the partial picture data DPi in order of the block number and are output.
In the above operation of the conventional digital data transmission apparatus 11, each length of the compressed picture data DCi produced in the quantizing and variable-length encoding section 17 depends on contents of a piece of corresponding partial picture data DPi. Therefore, the lengths of the compressed picture data DCi differ from each other, and the compressed blocks BCi are not fixed to the same size. Also, in cases where the compressed picture data DCi are transmitted, recorded or reproduced, it is required to prevent the occurring of errors in the compressed picture data DCi. Therefore, a group of compressed picture data DCi is equally divided in a prescribed length in the synchronizing-identifying-error correcting codes adding section 18, and the signal and codes SSi, IDi, PAi, NBi and ECi are added to each of the divided picture data having the prescribed length. As a result, the synchronizing blocks BSi are fixed to the same size.
Accordingly, the size of the synchronizing blocks BSi is not the same as those of the compressed blocks BCi. In other words, a piece of compressed picture data DCj (j is a positive integer) allocated in a compressed block BCj is sometimes divided into first and second parts, and the first part of compressed picture data DCj is allocated in a synchronizing block BSk (k is a positive integer) while the second part of compressed picture data DCj is allocated in a synchronizing block BSk+1 adjacent to the synchronizing block BSk. Also, a piece of divided picture signal is sometimes composed of a part of the compressed picture data DCj.
For example, as shown in FIG. 2(c), the synchronized picture data DS1 is composed of a synchronizing signal SS1, an identifying code ID1, an address pointer PA1 and a block number NB1, the compressed picture data DC1, a first part of the compressed picture data DC2 and an error correcting code EC1 in that order. The synchronized picture data DS2 is composed of a synchronizing signal SS2, an identifying code ID2, an address pointer PA2 and a block number NB2, the remaining part of the compressed picture data DC2, the compressed picture data DC3, a first part of the compressed picture data DC4 and an error correcting code EC2 in that order. The synchronized picture data DS3 is composed of a synchronizing signal SS3, an identifying code ID3, an address pointer PA3 and a block number NB3, a second part of the compressed picture data DC4 and an error correcting code EC3 in that order. The synchronized picture data DS4 is composed of a synchronizing signal SS4, an identifying code ID4, an address pointer PA4 and a block number NB4, the remaining part of the compressed picture data DC4, a part of the compressed picture data DC5 and an error correcting code EC4 in that order.
In cases where the compressed picture data DCi are decoded in the decoding section 21, it is required to start a decoding operation from each of head portions of the compressed picture data DCi for the purpose of correctly decoding the compressed picture data DCi. In other words, in cases where a decoding operation is started from a middle portion of a piece of compressed picture data DCj, the decoded part of the compressed picture data DCj does not make sense. Therefore, it is required to recognize each of the head portions of the compressed picture data DCi as a decoding operation starting point. To correctly decode the compressed picture data DCi, each of the compressed picture data DCi has a head address placed in its head portion. For example, as shown in FIG. 2(c), head addresses AHi are placed in the head portions of the compressed picture data DCi. Because the head addresses AH1, AH2 exist in the synchronizing block BS1, the compressed picture data DC1 and the first part of the compressed picture data DC2 are correctly decoded after the head addresses AH1, AH2 are recognized, and the remaining part of the compressed picture data DC2 is correctly decoded after the first part of the compressed picture data DC2 is correctly decoded. Also, because the head addresses AH3, AH4 exist in the synchronizing block BS2, the compressed picture data DC3 and the first part of the compressed picture data DC4 are correctly decoded after the head addresses AH3, AH4 are recognized, and the second and remaining parts of the compressed picture data DC4 are correctly decoded after the first part of the compressed picture data DC4 is correctly decoded. Also, because the head address AH5 exists in the synchronizing block BS4, the compressed picture data DC5 is correctly decoded after the head address AH5 is recognized. Therefore, in cases where the synchronized picture data DS1, DS2, DS3 and DS4 are reproduced from the recording medium 19 in that order, the compressed picture data DCi included in the synchronized picture data DSi can be correctly decoded.
Also, it is not necessarily sure that all of the compressed picture data DCi are transmitted through the transmission path 13, recorded in the recording medium 19 or reproduced from the recording medium 19. Also, a positional relationship among the partial picture data DPi are not necessarily the same as that among the compressed picture data DCi. For example, even though a piece of partial picture data DPj in a group of partial picture data DPi having a total length is placed at a particular position spaced at 1/10 of the total distance from a head portion of the group of partial picture data DPi, a piece of compressed picture data DCj obtained from the partial picture data DPj is not necessarily placed at the particular position spaced at 1/10 of a compressed total distance from a head portion of a group of compressed picture data DCi having the compressed total length. Therefore, it is required to add pieces of correspondence information at regular intervals to indicate the correspondence of the partial picture data DPi and the compressed picture data DCi. Also, it is required to add pieces of starting point information at the regular intervals to indicate the head addresses AHi of the compressed picture data DCi.
In the conventional digital data transmission apparatus 11, as shown in FIG. 3, an address pointer PAk allocated at an address pointer region RPk is added to the synchronized picture data DSk as the starting point information to preliminarily indicate a head address AHk of a piece of compressed picture data DCj which is first placed in a series of compressed picture data DCi having the head addresses AHi in the synchronized picture data DSk. Therefore, a decoding operation is started from the head address AHk to correctly decode the series of compressed picture data DCi. Also, a block number NBk allocated at a block number region RBk is added to the synchronized picture data DSk as the correspondence information to indicate the number of the compressed picture data DCj. Therefore, a position of the compressed picture data DCj in the synchronized picture data DSk is recognized. For example, in cases where the compressed picture data DC1, DC2 of the synchronizing block BS1 are decoded, the address pointer PA1 allocated at the address pointer region RP1 and the block number NB1 allocated at the block number region RB1 are read to recognize the head address AH1 of the compressed picture data DC1, and a decoding operation is performed from the head address AH1 of the compressed picture data DC1. Therefore, because an end address of the compressed picture data DC1 is recognized, the head address AH2 of the compressed picture data DC2 is automatically recognized, and a following decoding operation is performed from the head address AH2 of the compressed picture data DC2. In cases where the compressed picture data DC3, DC4 of the synchronizing block BS2 are decoded, the address pointer
allocated at the address pointer region RP2 and the block number NB2 allocated at the block number region RB2 are read to recognize the head address AH3, and a decoding operation is performed from the head address AH3 of the compressed picture data DC3. Therefore, because an end address of the compressed picture data DC3 is recognized, the head address AH4 of the compressed picture data DC4 is automatically recognized, and a following decoding operation is performed from the head address AH4 of the compressed picture data DC4. In cases where the compressed picture data DC5 of the synchronizing block BS4 is decoded, the address pointer PA4 allocated at the address pointer region RP4 and the block number NB4 allocated at the block number region RB4 are read to recognize the head address AH5, and a decoding operation is performed from the head address AH5 of the compressed picture data DC5.
Therefore, even though the synchronized picture data DS1, DS2, DS3 and DS4 are not reproduced from the recording medium 19 in that order, the head addresses AHi of the compressed picture data DCi can be recognized by reading the address pointers PAi and the block numbers NBi, and the compressed picture data DCi included in the synchronized picture data DSi can be correctly decoded. Accordingly, the decoding of the compressed picture data DCi can be performed regardless of whether or not the synchronized picture data DS1, DS2, DS3 and DS4 are reproduced in that order.
2.2. Problems to be Solved by the Invention
However, because any head address does not exist in the synchronizing block DS3, any address pointer is not written in the address pointer region RP3, and any block number is not written in the block number region RB3. That is, the address pointer region RP3 and the block number region RB3 are in an empty condition. Because the lengths of the compressed picture data DCi depend on contents of the partial picture data DPi while a group of compressed picture data DCi is equally divided in a prescribed length to produce the synchronized picture data DSi, the empty condition often occurs in the conventional digital data transmission apparatus 11. Therefore, because the address pointer regions RPi and the block number regions RBi set in the empty condition are not effectively utilized, there is a first drawback that a frame of picture data Dp input to the apparatus 11 cannot be compressed with a high efficiency.
Also, one or more pieces of important data DI included in a piece of converted picture data are generally placed in a front area of each of the encoded blocks BEi. In the important data DI, energy is concentrated. For example, in cases where the partial picture data DPi are converted into the converted picture data in the orthogonal converting section 16, converting coefficients for lower frequency components such as a DC coefficient are placed in the front areas of the encoded blocks BEi. Therefore, the important data DI represented by the converting coefficients are placed in a front area of each of the compressed blocks BCi. For example, as shown in FIG. 4A, the important data in the partial picture data DP1 are gathered in a front area AF1 of the compressed block BC1, the important data in the partial picture data DP2 are gathered in a front area AF2 of the compressed block BC2, the important data in the partial picture data DP3 are gathered in a front area AF3 of the compressed block BC3, the important data in the partial picture data DP4 are gathered in a front area AF4 of the compressed block BC4, and the important data in the partial picture data DP5 are gathered in a front area AF5 of the compressed block BC5. In this case, as shown in FIG. 4B, because a front area subsequent to the synchronizing signal region RSj and the identifying code region RIj in the synchronizing block BSj is not necessarily occupied by the important data DI.
In cases where the compressed picture data DCi recorded in the recording medium 19 are reproduced according to a special reproducing mode (or a high speed searching mode) in the conventional digital data transmission apparatus 11 utilized in a video tape recorder, all of the compressed data DCi included in a synchronizing block BSj are not necessarily reproduced. In this case, because one or more pieces of front data allocated in a front area following the synchronizing signal region RSj and the identifying code region RIj in the synchronizing block BSj are correctly reproduced at the highest probability, the front data are generally reproduced according to the special reproducing mode. However, because the front area is not necessarily occupied by the important data DI, the important data DI are not necessarily reproduced according to the special reproducing mode. Therefore, there is a second drawback that the reproduction according to the special reproducing mode cannot be performed at a high reproductivity.